The present invention relates to a sensor system employed in an optical disc drive for detecting a focusing error signal using a spot size detection method.
An optical system of an optical disc drive utilizing an optical disc, a magneto-optical disc or the like as a recording medium is generally provided with a laser source for emitting a laser beam, an objective lens for converging the laser beam on the recording medium to form a beam spot thereon, and a sensor system for receiving a beam reflected by the recording medium to detect a data signal, a focusing error signal and a tracking error signal.
A spot size detection method is conventionally used for detecting the focusing error signal. To perform the spot size detection method, the sensor system is provided with a collective lens for receiving and converging the reflected light beam, a hologram element which is inserted within the optical path of the converging beam to split the beam into two beams in a predetermined direction, and a first and a second sensors arranged in the predetermined direction for receiving the respective beams split by the hologram element.
The first and second sensors are arranged such that one of the sensors is located closer to the collective lens than a converging point of a beam incident thereon, and the other sensor is located farther from the collective lens than a converging point of the beam incident thereon.
An example of such a configuration will be described below. The hologram element divides an incident beam into a +1st diffraction light beam and a -1st diffraction light beam in a first direction. Further, the hologram element functions as a positive lens for the +1st order diffraction light beam, and as a negative lens for the -1st order diffraction light beam. With this constitution, the +1st order diffraction light and the -1st order diffraction light are converged at different positions having different distances from the hologram. The +1st order diffraction light is received by the first sensor which is arranged farther from the collective lens than the converging point of the +1st order diffraction light beam. The -1st order diffraction light is received by the second sensor which is arranged closer to the collective lens than the converging point of the -1st order diffraction light beam. In this configuration, 0th order light beam is not utilized (i.e., not received by the first and second sensors).
The sizes of the beam spots formed on the first and the second sensors change depending on a focusing condition of the objective lens. Therefore, by detecting a difference in size of the beam spots on the first and second sensors, the focusing error signal indicative of the focusing condition of the objective lens can be detected.
In this specification, directions with respect to orientation of sensors will be indicated by directions X and Y. The direction X and the direction Y are defined as follows.
(a) Direction X: the direction X corresponds to a radial direction of the disc at a point where a beam spot is formed and reflected (i.e., a relationship between the direction X with respect to the beam spot formed on a sensor is similar to a relationship between a radial direction of the optical disc with respect a point at which the beam spot is formed); and PA1 (b) Direction Y: the direction Y corresponds to a direction tangential to a track of the disc at a point where a beam spot is formed and reflected (i.e., a relationship between the direction Y with respect to the beam spot formed on a sensor is similar to a relationship between an extending direction of a tangential line to a track of the disc at a point where a beam spot is formed). PA1 (a) a focusing error signal based on output signals of each of light receiving areas of the second and third sensors, in accordance with a spot size detection method, and PA1 (b) a tracking error signal based on output signals of the light receiving areas of the first sensor, in accordance with a push-pull method.
In addition to the above, in order to obtain a tracking error signal based on the signals output by the first and second sensors, the hologram element may be formed to split an incident beam in the first direction.
For example, as shown in FIG. 1, the first and second sensors area arranged in the X direction, substantially symmetrical with respect to a 0th order light beam, and each of the first and second sensors has four light receiving areas (first sensor: areas A-D; second sensor: areas E-H) which are divided by lines extending in the Y direction.
The focusing error signal FE and a tracking error signal TE are represented by the equations below: EQU FE=Sa-Sb-Sc+Sd-Se+Sf+Sg-Sh;
and EQU TE=Sa+Sb-Sc-Sd-Se-Sf+Sg+Sh,
where, signals output from the areas A-H are represented by Sa-Sh, respectively.
In the above-described conventional sensor system, however, when the beam spot formed on the disc moves in a transversal direction of the tracks of the disc when data search or the like is performed, even if the objective lens focuses on the disc (i.e., even if there is no focusing error), changes of brightness occur at a central portion, in the X direction, of the spots on the first and second sensors, and the focusing error signal may fluctuate.
FIG. 2 shows a graph indicating changes of the tracking error signal (solid line, 1/10 scale) and the focusing error signal (broken line) when the beam spot on the disc moves from a center of a land (X'=0) to a next land (X'=1) using an objective lens whose NA (numerical aperture) is 0.6, a collective lens whose focal length is 27 mm, and sensor systems shown in FIG. 1. In the graph, X'=0.5 represents a center of a groove which is formed between adjacent lands.
As shown in FIG. 2, when the spot on the disc moves in the radial direction (X') of the disc, the focusing error signal, which should have a flat characteristic on the graph, contains a noise component. In this specification, the noise component included in the focusing error signal which is generated when the spot is moved in the radial direction of the disc is referred to as a T/F (Tracking/Focusing) cross-talk. The T/F cross-talk appears as brightness change at a predetermined area on the spot formed on the sensors.